Final answer:
The most commonly used unit of time by physicists is the second, which has been precisely defined in terms of cesium atom vibrations.
Step-by-step explanation:
The unit of time most commonly used by physicists and other scientists is the second. Historically, the second was defined as 1/86,400 of a mean solar day. However, due to the gradual slowing of Earth's rotation leading to a longer solar day, a need arose for a more precise measurement. In 1967, the second was redefined to be more accurate, established in terms of the constant vibrations of cesium atoms. A second is now defined as the duration of 9,192,631,770 vibrations of the cesium atom, a stable and consistent physical phenomenon. These vibrations are what constitute the measurement in cesium atomic clocks, making the second a highly precise unit for time measurement.